1,6-hexanediol is a useful substance used as a raw material of polyester resins, urethane foam, urethane paint and adhesives. For example, it can be used directly as a chain extender when used as the raw material of polyurethanes, or can be used as a soft segment by using in the production of polycarbonate diols and polyester polyoles.
Although cyclohexanone and/or cyclohexanol, which are useful as raw materials in the synthesis of ε-caprolactam, are produced industrially by aerobic oxidation of cyclohexane, 1,6-hexandiol is produced by esterifying a mixture of carboxylic acids, including glutaric acid, adipic acid, 6-hydroxycaproic acid and so on produced as by-products during aerobic oxidation of cyclohexane, with alcohol followed by hydrogenation to obtain 1,6-hexanediol that is separated by distillation (Patent Documents 1 and 2).
Since the 1,6-hexanediol obtained by the aforementioned method contains impurities such as 1,4-cyclohexanediol, 1,5-hexanediol, 1,2-cyclohexanediol, 1,7-pentanediol, 1,5-pentanediol and high boiling point components, if, for example, a polycarbonate diol is produced using 1,6-hexanediol for the raw material, and this is then used as a raw material to carry out a urethanation reaction, it was found that the polymerization rate is slow, an adequate molecular weight cannot be obtained, and that similar problems occur even in the case of using directly as a chain extender in a urethanation reaction. In addition, there was also the problem of similar effects appearing on the polymerization rate during polyester production as well.
With respect to removal of the impurity, 1,4-cyclohexanediol, Patent Document 3 describes a method for converting the aforementioned mixture of carboxylic acids to cyclohexanone and cyclohexanol by preliminary hydrogenation following esterification of the carboxylic acid mixture with an alcohol, while Patent Document 4 describes a method for obtaining an ester essentially free of 1,4-cyclohexanediol by distillation.
However, the method employing preliminary hydrogenation had the problem of inadequate product purity. In addition, although in the method involving ester distillation it is preferable in that the esters are monomers, in actuality, many of the active ingredients of the carboxylic acid mixture become oligomer esters as a result of concentration following extraction with water, thereby requiring not only esterification of the carboxylic acid monomers but also depolymerization of the oligomer esters in order to obtain 1,6-hexanediol at high yield and high purity. However, in the production processes of the prior art, since esterification of carboxylic acid monomers and depolymerization of oligomer esters are carried out simultaneously, Lewis acid catalysts and basic catalysts effective in depolymerization are deactivated by the water formed by esterification of carboxylic acids and residual carboxylic acids, thereby resulting in the problem of considerable time being required for depolymerization and the problem of corrosion of the reaction vessel by water, carboxylic acids and acid catalysts (such as mineral acids). In addition, although esterification and depolymerization are equilibrium reactions, there was also the problem in being unable to increase the equilibrium reaction yield due to the effects of the water formed. Moreover, considerable equipment was required to separate the esters, water, alcohols and other components formed as a result of esterification.
[Patent Document 1] U.S. Pat. No. 3,524,892
[Patent Document 2] U.S. Pat. No. 3,268,588
[Patent Document 3] Japanese Unexamined Patent Publication No. S51-108040
[Patent Document 4] Japanese Unexamined International Publication No. 2000-505468